Electronic commerce is moving towards a vision of web service based interactions, where businesses use web services to interact with each other dynamically. For example, a service provided by one business could spontaneously decide to engage a service provided by another business.
In order for these services to interact with each other dynamically, they must be able to do three fundamental things. First, businesses (e.g., clients) must be able to discover services provided by other businesses. Second, a service must be able to describe its abstract interfaces and protocol bindings, so that clients can figure out how to invoke the service. Third, a service must be able to describe the kinds of interactions that it supports (e.g., that it expects clients to login before they can request a catalog), so that the service can engage in complex exchanges with its clients.
Universal Description Discovery and Integration (UDDI) specifications address the first problem by defining a way to publish and discover information about web services. Web Services Description Language (WSDL) addresses the second problem by defining a set of XML schemas for describing the interface and protocol bindings of network services.
The last problem is not addressed by either UDDI or WSDL. Neither UDDI nor WSDL currently addresses the problem of how a service can specify sequences of message exchanges (interactions) that it supports.
Web services are much more loosely coupled than traditional distributed applications. This difference impacts both the requirements and usage models for web services. Web services are deployed on behalf of diverse businesses, and the programmers who implement them are unlikely to collaborate with each other during development. However, a purpose of web services is to enable business-to-business interactions. Therefore, web services must support flexible and dynamic interactions. Web services should also be able to discover new services and interact with them dynamically without requiring programming changes to either service.